Quick answer
What is a thyroid function test?
A thyroid function test is a blood test that checks TSH and sometimes T4, used to diagnose an underactive or overactive thyroid. No fasting is needed beforehand. It is often requested when someone has symptoms such as tiredness, weight change or changes in heart rate.
How thyroid hormones work
TSH is released by the pituitary gland and regulates how much hormone the thyroid produces. The thyroid hormones T4 and T3 control the body’s metabolism, affecting things like energy levels, weight and heart rate.
Underactive thyroid (hypothyroidism)
An underactive thyroid typically shows up as a high TSH level alongside a low T4 level on blood tests. Levothyroxine replacement is the standard treatment and is usually taken long term.
Overactive thyroid (hyperthyroidism)
An overactive thyroid typically shows a low TSH level alongside high T4 or T3 levels. Treatment options include carbimazole, radioiodine treatment or surgery, depending on the cause and how severe it is.
Monitoring thyroid function over time
Once treatment has started, doses are adjusted based on repeat blood tests until levels settle. After that, an annual review is usually enough to check that thyroid function remains stable.
Common questions about the thyroid function test
- What are the symptoms of an underactive thyroid?
- Symptoms of an underactive thyroid can include tiredness, weight gain, cold intolerance, dry skin and constipation. A thyroid function test can help confirm whether these symptoms are linked to your thyroid.
- What are the symptoms of an overactive thyroid?
- Symptoms of an overactive thyroid can include weight loss, tremor, heat intolerance, palpitations and anxiety. These symptoms can overlap with other conditions, so a blood test is used to confirm the diagnosis.
- Do you need to fast before a thyroid function test?
- No fasting is needed before a thyroid function test. If you are already taking levothyroxine, you should take it after your blood has been drawn rather than before.
- How is thyroid function monitored during pregnancy?
- Thyroid hormone targets are stricter during pregnancy, so monitoring tends to be more frequent. Specialist input is typically involved to keep levels within the recommended range.